1. Field of the Invention
The present invention relates to satellites. More specifically the present invention relates to satellite station keeping.
While the present invention is described herein with reference to illustrative embodiments for particular applications, it should be understood that the invention is not limited thereto. Those having ordinary skill in the art and access to the teachings provided herein will recognize additional modifications, applications and embodiments within the scope thereof and additional fields in which the present invention would be of significant utility.
2. Description of the Related Art
Synchronous satellites orbit the earth with the same revolution rate as that of the earth. Accordingly the satellite appears above a fixed point on the earth. Hence, synchronous satellites are also referred to as "geostationary" satellites and operate within a "stationary" orbit. Synchronous satellites are useful for many applications including weather and communications applications.
It is generally well known in the art that various forces act on synchronous satellites which act to move the satellite out of stationary orbit. These forces are due to several sources including the gravitational effects of the sun and moon, the elliptical shape of the earth and solar radiation pressure. To counter these forces, synchronous satellites are equipped with propulsion systems that are fired at intervals in order to maintain station at a desired orbit. This requires control of the inclination, eccentricity and drift of the satellite. Inclination is the north-south position of the satellite relative to the earth's equator. Eccentricity is the measure of the non-circularity of the satellite orbit. That is, the measure of the variation of the distance the satellite is from the earth as the earth and satellite rotate. Finally, drift is the position of the satellite in an east-west direction relative to a sub-satellite point on the earth.
Station keeping was first achieved with the Syncom III satellite, a Hughes Aircraft Company spin-stabilized communications satellite launched by NASA in 1964. Current satellites are either spin-stabilized or three-axis stabilized satellites. Spin-stabilized satellites use the gyroscopic effect of the satellite spinning to help maintain the satellite orbit. For certain applications, the size of the satellite militates in favor of a three-axis stabilization scheme.
Current three-axis stabilized satellites use liquid propulsion systems for station keeping. One set of thrusters are used for controlling the inclination while a second set of thrusters are used for controlling the drift and eccentricity of the satellite. As the cost of satellite propulsion systems is directly related to the number of thrusters required for station keeping, there is an ongoing need in the art to reduce the number of thrusters required for satellite propulsion and station keeping.
Further, liquid propulsion systems have a limited life because of the limited supply of fuel on board the satellite. Thus, there is an additional need in the art for a satellite thruster for use on three-axis stabilized satellites with longer life than current liquid propulsion thrusters.